2017 iMac Retina 5k running super slow

Where did you buy that Crucial BX500 2T SSD...?

Currently that Crucial BX500 SSD is slower than an old worn out HDD.

If Enabling Trim doesn't help?

see > https://www.akitio.com/faq/377-how-to-enable-trim-on-macos

Then you need to return that Crucial BX SSD to the seller and buy something else like the Crucial MX, a WD, a PNY or perhaps a Samsung SSD.

What sled/adapter and cable did you use to convert from the 3.5” HDD to the 2.5” SSD…?

Those BX500 SSDs are known to have problems in Mac’s.

The preferred 2.5” SSD upgrade kit comes from OWC.

https://www.amazon.com/OWC-2012-2015-Converter-Installation-OWCK27IM12HE500/dp/B076LJFND3/ref=asc_df_B0026SB27Y?

Is that an Apple Authorized Service Center...?

If it is an authorized service, then take it back and have them make it right.

If it is not, then there is no telling what other short cuts they took besides using the BX500.

Using the proper sled/adapter, cable and display reseal kit is as important as using a good SSD.

If you buy the OWC kit, it will come with everything needed plus an OWC SSD.

Over the years I've used both CS1311 and CS900 PNY SSDs in more than a dozen Mac's. They are all still preforming and at the advertised Read/Write speed.

EtreCheckPro is an excellent tool for finding software and hardware issues.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post back here with your Report, as per > How to use the Add Text Feature When Post… - Apple Community

alansellsre wrote:

nothing connected to the Mac. honest that seem odd that the old HD was doing this, and now a new SSD was installed and its acting the same?

If the old hard drive and the new SSD are both failing, that might suggest that that there is a problem in the hard disk controller circuitry.

Usually failures from an old hard drive would be coming from the hard drive itself, but if the controller has gone bad, it could be throwing errors regardless of what drive you attached to it. Fixing that might be difficult, since that controller would probably be soldered onto the motherboard.

You could try getting an external SSD, making it your startup drive, and leaving the internal drive unused. That would not be a guaranteed fix, because the internal drive or controller might throw errors that would crash the operating system, or slow things down, even when you were not using the drive.

I don't mind buying another SSD to try it but could their be something in the power supply or another board causing these HD's to fail?

Quite possibly.

+1 on EtreCheck, please post your computer’s report.

Check again for a local authorized service provide.

Find Locations

iMac DIY is not easy.

Watch the "Hard Drive Install" video before you buy or do anything.

https://eshop.macsales.com/installvideos/imac-27-inch-5k-late-2015/

It appears the brand new SSD is failing. The report shows:

This computer has a hard drive that appears to be failing.

If there are any hard disks attached to the computer, shut down the computer, physically disconnect the HD and then restart and test. If your performance returns then you have found the culprit.